For the purpose of improving strength and toughness of a hot-rolled steel plate or other heated metal plate, it is the conventional practice to eject cooling water onto the upper and lower surfaces of the heated metal plate horizontally moving in the longitudinal direction thereof to cool the metal plate to a prescribed temperature.
The conventional apparatus for cooling a heated metal plate to a prescribed temperature comprises upper cooling water ejecting nozzles for ejecting cooling water substantially vertically onto the upper surface of the metal plate, an upper nozzle header for supplying cooling water to the upper cooling water ejecting nozzles, lower cooling water ejecting nozzles for ejecting cooling water onto the lower surface of the metal plate, and a lower nozzle header for supplying cooling water to the lower cooling water ejecting nozzles.
As shown in FIG. 1, the plurality of upper cooling water ejecting nozzles 2 are arranged spaced apart from each other at prescribed intervals, above the heated metal plate (not shown) in the width direction of the heated metal plate, and eject cooling water supplied from the upper nozzle header 1 substantially vertically in the form of a lamination onto the upper surface of the metal plate.
The plurality of lower cooling water ejecting nozzles (not shown) are arranged spaced apart from each other at prescribed intervals below the heated metal plate in the width direction thereof, and eject cooling water supplied from the lower nozzle header (not shown) substantially vertically in the form of a mist onto the lower surface of the metal plate.
In the above-mentioned apparatus for cooling the heated metal plate, it is very important, with a view to reducing strain and other inconveniences produced in the metal plate, that the upper cooling water ejecting nozzles 2 and the lower cooling water ejecting nozzles have substantially the same cooling abilities.
For this purpose, it was the usual practice, in the above-mentioned apparatus for cooling the heated metal plate, to increase the flow rate of cooling water supplied from the lower nozzle header to the lower cooling water ejecting nozzles to from 2.0 to 2.5 times greater than the flow rate of cooling water supplied from the upper nozzle header 1 to the upper cooling water ejecting nozzles 2.
The reason is as follows. Cooling water after ejection from the lower cooling water ejecting nozzles onto the lower surface of the heated metal plate leaves immediately the lower surface and drops down, whereas cooling water after ejection from the upper cooling water ejecting nozzles onto the upper surface of the metal plate stays for a while on the upper surface, and consequently brings about a secondary cooling effect. Therefore, if cooling water ejected onto the upper surface of the heated metal plate has the same flow rate as that of cooling water ejected onto the lower surface thereof, the upper surface would be more easily cooled than the lower surface.
However, ejecting cooling water in a large quantity onto the lower surface of the metal plate as mentioned above is not desirable from the point of view of resource saving.
A cooling apparatus solving the above-mentioned problem is disclosed in Japanese Patent Provisional Publication No. 55-156,612 (hereinafter referred to as the "prior art"). The principle of the apparatus for cooling a heated metal plate of the prior art is described below with reference to FIG. 2.
As shown in FIG. 2, a heated metal plate 3 is laid horizontally. A water tank 4 comprising a bottom wall 4a and side walls 4b, for receiving cooling water, is arranged below the heated metal plate 3. The water tank 4 has a size sufficient to collect the total amount of a jet stream described later. The bottom wall 4a of the water tank 4 is provided with a plurality of lower cooling water ejecting nozzles 5 substantially vertically arranged spaced apart from each other at prescribed intervals in the width direction of the heated metal plate 3. The uppermost end of each lower cooling water ejecting nozzle 5 is located under the surface of cooling water received in the water tank 4. A lower nozzle header 6 for supplying cooling water to the lower cooling water ejecting nozzles 5 is connected to these nozzles 5. A plurality of upper cooling water ejecting nozzles (not shown) similar to those shown in FIG. 1 are arranged above the heated metal plate 3 spaced apart from each other at prescribed intervals in the width direction of the heated metal plate 3 and eject cooling water substantially vertically onto the upper surface of the heated metal plate 3.
In the above-mentioned apparatus for cooling a heated metal plate of the prior art, when cooling water is supplied from the lower nozzle header 6 to the lower cooling water ejecting nozzles 5 in the state of the water tank 4 filled with cooling water, both cooling water from the lower cooling water ejecting nozzles 5 and cooling water received in the water tank 4 are ejected in the form of a jet stream 7 substantially vertically onto the lower surface of the heated metal plate 3, and thus the heated metal plate 3 is cooled to a prescribed temperature. The jet stream 7 after ejection onto the lower surface of the heated metal plate 3 is totally collected in the water tank 4. Cooling water in an amount substantially equal to that of cooling water supplied from the lower nozzle header 6 to the lower cooling water ejecting nozzles 5 overflows from the water tank 4.
According to the above-mentioned cooling apparatus of the prior art, it is possible to cool the lower surface of the heated metal plate by cooling water at a flow rate several times as large as that of cooling water from the lower cooling water ejecting nozzles 5, thus remarkably improving the cooling ability of the cooling apparatus. In addition, since the jet stream 7 after ejection onto the lower surface of the heated metal plate 3 is totally collected into the water tank 4, only the amount of cooling water supplied from the nozzle header 6 to the lower cooling water ejecting nozzles 5 is consumed as the overflow from the water tank 4. Consumption of cooling water is thus largely reduced.
The prior art described above has however the following problems:
When the position of the uppermost end of the lower cooling water ejecting nozzle 5 and the flow rate of cooling water supplied to the nozzle 5 are kept constant, the flow rate of the jet stream 7 varies in response to the variation of the surface level of cooling water received in the water tank 4. More specifically, if the distance between the lower surface of the heated metal plate 3 and the surface level of cooling water in the water tank 4 is kept constant, the ability to cool the heated metal plate 3 depends upon the flow rate of the jet streams 7. It is therefore necessary to keep always constant the surface level of cooling water in the water tank 4 in order to uniformly cool the heated metal plate 3. However, dropping of the jet stream 7 after ejection onto the lower surface of the heated metal plate 3 into the water tank 4 causes considerable up and down wavy movements of the surface of cooling water in the water tank 4, and the uppermost end of the lower cooling water ejecting nozzle 5 may sometimes be even exposed above the water surface. Furthermore, when the jet stream 7 falls into the water tank 4 as mentioned above, innumerable bubbles are produced on the surface of cooling water in the water tank 4, and these bubbles are entangled into the jet stream 7, thus deteriorating the cooling ability. Thus, according to the prior art, the heated metal plate cannot be uniformly and efficiently cooled.
Another method has recently been developed which comprises subjecting a heated steel plate immediately after hot rolling to an online controlled cooling to minimize alloy elements, and thus manufacturing a high-strength steel plate excellent in toughness. In this method, it is necessary to control the cooling rate of the heated steel plate in response to the thickness and other particulars of the plate in order to manufacture a steel plate with a desired quality, and a wider range of control of the cooling rate permits manufacture of more kinds of steel plate. However, if the flow rate of cooling water from the lower cooling water ejecting nozzles 5 is reduced to decrease the cooling rate, the jet stream 7 may not reach the lower surface of the headed steel plate, and if the flow rate of cooling water from the lower cooling water ejecting nozzles 5 is increased to increase the cooling rate, on the contrary, the jet stream 7, reaching the lower surface of the heated steel plate, is ejected in a state close to mist onto the surface of the heated steel plate, and the cooling rate cannot be increased. Thus, according to the cooling apparatus of the prior art, the cooling rate of the heated metal plate cannot be controlled over a wide range.
Under such circumstances, there is a demand for the development of an apparatus which permits, when cooling a heated metal plate horizontally lying above a water tank to a prescribed temperature by means of a jet stream produced by cooling water from lower cooling water ejecting nozzles arranged in the water tank and cooling water received in the water tank, uniform and efficient cooling of the heated metal plate and also control of the cooling rate over a wide range. However, such an apparatus is not as yet proposed.